While hydrogen has wide potential as a fuel, a major drawback in its utilization, especially in mobile uses such as the powering of vehicles, has been the lack of an acceptable lightweight hydrogen storage medium. Certain materials and alloys in solid state have the ability to absorb and desorb hydrogen. These materials have been considered as a possible form of hydrogen storage, due to their large hydrogen storage capacity. Storage of hydrogen as a solid hydride can provide a greater volumetric storage density than storage as a compressed gas or a liquid in pressure tanks. Also, hydrogen storage in a solid hydride presents fewer safety problems than those caused by hydrogen stored in containers as a pressurized gas or a cryogenic liquid. Solid-phase storage of hydrogen in a metal or alloy system works by absorbing hydrogen through the formation of a metal hydride under specific temperature/pressure or electrochemical conditions, and releasing hydrogen by changing these conditions (usually under heating). Metal hydride systems have the advantage of high-density hydrogen storage for long periods of time.
Metal hydrides suffer some drawbacks though. The majority of metal hydrides are sensitive to oxygen and moisture. Exposure to air or moisture will result in an exothermic chemical reaction, causing the material to lose its hydrogen storage capacity and potentially creating a fire hazard. Additionally, adsorption and desorption of hydrogen occurs at elevated temperatures, requiring that the metal hydrides be surrounded by a heat transfer medium. Metal hydride is usually stored in a storage vessel thermally integrated with a heat exchanger, preferably an internal heat exchanger, to provide the most efficient heat transfer. The heat transfer mediums currently in use are metal fins or aluminum foam. To release hydrogen, it is necessary to heat the whole storage vessel thus increasing heat losses. Because on-board charging is not considered viable due to the high hydrogen pressure required and fast heat release, this hydrogen storage system requires that the storage vessel containing metal hydride and the heat exchanger be exchanged when the metal hydride is exhausted or contaminated and can no longer be effectively recharged with hydrogen. Container exchange is a labor intensive process that demands the redesign of cars and refueling infrastructure, and will hinder public acceptance of the technology in vehicles. Additionally, the container exchange would likely require container standardization across a wide assortment of vehicles—a daunting prospect. Consequently there is a need for a rapid method of refueling for hydrogen based energy systems and delivery of hydrogen to the fuel cell on demand, without heating of the whole storage tank. In fact, this need has been recognized by the U.S. Department of Energy in that they have set an ambitious goal of a three minute refueling time. It has been proposed to use a liquid organic hydrogen carrier that is capable of releasing hydrogen at heating, and transforming to a dehydrogenated liquid form. However, these organic carriers have low hydrogen content. Accordingly, there exists a need for high capacity hydrogen storage materials that facilitate refueling.